This invention relates to new compositions and process for the preparation of modified polyimide foams, possessing a density of 0.6 to 1.0 lb/ft.sup.3, which are especially useful where resistance to moisture and flame is desirable. More specifically these foams can be used in adverse environments including steam pipe insulation and insulation of tanks with a steam pressure of 5-40 psi without degradation of the polymer chain.
The technology of the previous art is limited to the concept of selecting compositions and processes to form prepolymers which upon heating produce polyimide foams possessing the properties of resistance to high temperature and non-burning without emission of smoke.
The technology of the previous art, however, did not address the problem of durability of the foams at ambient and adverse conditions. These previous foams, when exposed to high humidity environments, which are encountered in normal use, start to decompose and become brittle and powdery. The result is that all the properties, including flammability properties, are degraded and the foams do not perform any longer in the applications they were originally intended for. This is a serious problem because these polyimide foams are now being considered for use in insulation of aircraft, naval vessels and other transportation vehicles, without the full knowledge that catastrophic failure of the insulation could occur after the polyimide foam is degraded.
The previous art is disclosed in, inter alias, U.S. Pat. Nos. 3,506,583, 3,726,831, 3,726,834, 4,070,312, 4,296,208, 4,305,796, 3,554,939, 3,554,935. These patents not only describe various methods for producing polyimide foams but also processes for scaling up by using spray drying and microwave heating techniques.
Although some patents allege that the foams produced are hydrolytically stable, standard tests proposed by the U.S. Navy show degradation when tested according to ASTM. Therefore, there is a definite need for a composition and a process for producing improved temperature and humidity resistant foams. The foams described can be prepared directly by heating a composition prepared by mixing a solution of an ester of benzophenonetetracarboxylic acid, caprolactam and a polyamine by using a solvent system comprising a low boiling point alcohol in combination with a higher boiling alcohol. The compositions and processes are fully described in the examples.
We believe that it is the caprolactam which imparts these unique properties to our product, but do not have a simple explanation. The use of caprolactam in modified polyimide products is not new. U.S. Pat. No. 3,238,181, issued Mar. 1, 1966 describes heat resistant polymers containing recurring cyclic imide groups connected by amide or ester linkages for making coatings. This patent discloses materials useful in our invention but does not disclose methods to produce foams.
Another invention made by one of the inventors of this application, U.S. Pat. No. 3,505,272 describes methods to make copolymer adhesive systems by reacting caprolactam with a dianhydride and a glycol.
U.S. Pat. Nos. 4,183,838 and 4,183,839 by the same inventor describe and claim useful coatings based upon the reaction of a dianhydride with caprolactam.
In all of the cited patents, which incorporate caprolactam, an N-substituted imide intermediate is formed by the reaction of the dianhydride with the caprolactam which is then polymerized with specific reactants to form specific end products. However, these processes and compositions will not produce foams. Therefore, a further need exists for a process which yields modified polyimide foams possessing a density in the range of 0.6 to 1.0 lb/ft.sup.3, and resistance to steam and to high temperature and improved thermal insulating properties.
In the processes for the production of polyimide or modified polyimide resins now in use, the resin solution in a solvent is fed into a spray dryer and the powder resin is collected at the bottom of the dryer. Many resins, including polyimide and modified polyimide resins when spray dried from a single solvent system form a sticky powder due to the high concentration of wet resin in the powder granulates. The removal of the last traces of solvent from these wet powder resins is difficult and costly.
These wet powders must be dried in a thermal oven at relatively low temperature, in a microwave oven or in vacuum ovens to prevent polymerization. Moreover it is difficult to recover all the powder charged to the dryer due to physical losses during the operation. It is important that these powders be completely dried before they are processed to finished products or poor quality materials are obtained. The most important product produced from the polyimide precursor are polyimide foams.
The previous spray drying methods used to produce polyimide or modified polyimide precursors are disclosed, inter alia, in U.S. Pat. Nos. 3,506,583, 4,296,208, 4,305,796, 4,218,555, 4,255,313 and 4,251,418.
According to these previous disclosures it is possible to completely dry the resin precursors by using high inlet and high outlet temperatures during the spary drying process, however, when powders produced at these conditions are formed by thermal or microwave processes the resulting foams are extremely poor in quality. For an example, when the inlet and outlet temperatures of the spray dryer are 15.degree. to 30.degree. C. and 30.degree. to 60.degree. C. higher than the boiling point of the solvent to be evaporated, respectively, the resulting powder is very dry, but yields foams with large flaws throughout. We believe that powders produced at these conditions are partially polymerized and yield poor quality foams.
W. R. Boram, in U.S. Pat. No. 3,506,583 discloses that the liquid polyimide foam precursors in an alcohol solution can be spray dried since the solvents used have low boiling points and they do not require the use of catalytic solvents.
U.S. Pat. No. 4,296,208 discloses and claims a process of spray drying polyimide foam precursors in an alcohol solution and limits the outlet temperature to a maximum of 80.degree. C. U.S. Pat. No. 4,305,796 also claims the use of spary drying the liquid polyimide precursor in an alcohol solution. The outlet temperature of the chamber being limited to 80.degree. C. maximum. P. A. Antonopolos, in U.S. Pat. No. 4,218,555 claims the spray drying of modified polyimide resins precursors in an alcohol solution using the minimum amount of heat to vaporize all the solvent.
Similarly, U.S. Pat. Nos. 4,255,313, 4,251,418, 4,299,750 and 4,251,420 disclose the use of spray drying modified polyimide precursors in an alcohol solvent. These patents disclose spray drying processes of polyimide precursors that are completely different than that of the present invention and limit the outlet temperature of spray drying to 80.degree. C. maximum. These precursors produce foams substantially different than those of the present invention in that the foams have poor hydrolytic stability and decompose in use.
The Antonopolos modified polyimide precursors are also different from those of the present invention and may or may not produce flexible foams upon heating